1. Technical Field
The disclosed embodiments relate to charge pumps, and more particularly to charge pumps used in Phase-Locked Loops (PLLs).
2. Background Information
PLLs are used in many applications, including use in the local oscillators of cellular telephone receivers and transmitters. PLLs, such as those in the local oscillators of cellular telephones, are often implemented using digital Complementary Metal-Oxide-Semiconductor (CMOS) fabrication processes and often involve ring-type Voltage Controlled Oscillators (VCOs) built with inverter delay elements. Such VCOs typically have high gain when implemented using nanometer CMOS processes. A PLL that uses a high-gain VCO can be stabilized by increasing the loop filter capacitance or by reducing the charge pump current. For a given value of loop filter capacitance, PLLs using these VCO topologies are restricted to using a low value of charge pump current in order to compensate for the large VCO gain. It is challenging to build a charge pump that operates at low current levels with desirable operating characteristics. Charge pump performance at low current levels may be sensitive to second order effects and may suffer due to unwanted coupling of bias nodes to other nodes.
FIG. 1 (Prior Art) is a circuit diagram of a typical charge pump 1 within a PLL. Charge pump 1 receives an up charge pump control signal (UPB) on an input node 2 and a down charge pump control signal (DN) on an input node 3. Signal UPB controls a charge pump up current (IUP), and signal DN controls a charge pump down current (IDN). Nodes 5 and 6 are bias nodes. Charge pump 1 supplies charge onto charge pump output node 4 by conducting a flow of up current (IUP), and by removing charge from charge pump output node 4 by conducting a flow of down current (IDN). The resulting charge pump current pulse train signal ICP is then typically supplied to a loop filter which integrates the pulses thereby generating a Direct Current (DC) signal. The DC signal is supplied to and controls the VCO of the PLL. A charge pump such as charge pump 1 is seen to exhibit non-linear operating characteristics and undesirably low gain when operated at low current levels.